The challenge part: A good deal of the troubles with the Spruce Goose stemmed from the personal friction of the two men cobbled together for its completion:inventor Howard Hughes, notorious for his micromanaging and shipyard magnate Henry Kaiser, nicknamed 'Hurry up Henry' for a reason.
So my scenario for 'saving' the HK-1: once the detailed plans are drawn, Kaiser buys out Hughes and completes the aircraft on its own. (In order for Hughes to agree, let's say he gets another pipe-dream that hoggs all its attention: May be a jet engine, may be a new movie project, may be a new girlfriend....) Anyway, Kaiser, the man that just built 52 escort aircraft carriers in 52 weeks, manages to not only finish the first HK1 by December 1944, but also build a new one every two weeks until VJ day...
Part 2: As for the impact of the plane: Surprisingly little as the US by now has established a smooth running supply line to Europe as well as to the pacific. Also the U-boats in the Atlantic... The main reason for the HK1 to be developed turned out to be less and less of a threat once Kaiser's 52 escort carriers became operational and learned how to put a safe screen around the convoys. I the end the only thing I can see the spruce goose being good at is at sacrificing half of her payload for extra fuel and then flying the New-York Ireland route nonstop as a fast courier-transport. A second route San Francisco to Hawaii might be interesting too.